The present invention relates to a method for determining the refractive power of a transparent object, e.g., a windshield, a visual aid, a cockpit glazing, a helmet visor, or the like, and to a corresponding device.
The maximum refractive powers permitted in the windshield (WSS) are defined in statutory requirements such as ECE R43 or Directive 2001/92/EC and must be complied with by the manufacturers of the glass panes or the corresponding vehicles.
In order to measure the refractive power of such glass panes, a method is known in which a grating having a multiplicity of circular disks disposed at fixed distances of, e.g., 24 mm, and each having a diameter of, e.g., 8 mm, is projected through a windshield onto a screen. In this method, the magnitude of the deformation of the projected bright circular disks is a measure of the spatial distribution of refractive powers present in the glass pane. In the projection through a glass pane with a non-spherical lens (general case), every circular disk of the grating is distorted into an ellipse rotated through an angle φ. In other words, the circular disk is, e.g., compressed in one direction and is lengthened in the other direction. The deformation of the circular disk 7 into an ellipse 9 is shown in FIG. 1. When spherical lenses are used, the circular disks are enlarged or reduced in size.
Using the known circle grating method, it is therefore possible to determine the extreme values of the refractive powers directly in the points of the windshield selected for the projection of the circular disks by measuring the axes of the ellipse or the diameter of the circle. The position of the non-spherical lens is given by the angle φ, which can also be measured. The lens coordinate system is rotated through the angle φ relative to the measurement coordinate system.
The known method has the disadvantage, however, that the circular disks have a certain, predetermined distance from one another, as indicated above, i.e., in order to measure the entire windshield, a plurality of measurements must be carried out in which the grating is displaced relative to the windshield. In so doing, investigators attempt to manually fix the position of the circular disks in such a way that the circular disks detect the regions having the extreme values of the refractive powers. A template on the screen is subsequently used to determine whether the refractive power of the windshield deviates from the requirements or not at the corresponding points on the windshield. This is a highly complex procedure with a high susceptibility to error.